Sulphuric acid manufacture



Patented Jan. 28, 1936 UNITED STATES PATENT OFFICE mesne assignments, to American Cyanamid & Chemical Corporation, a corporation of Delaware Application September 21, 1933,-Serial No. 690,366

4 Claims.

My-invention relates more particularly to the catalytic oxidation of sulphur dioxide to sulphur trioxide by passing the gas mixture produced from burning the sulphur containing materials over suitable catalytic materials without allowing the temperature of the gases throughout the process to fall materially below that required for an approximate 97-98% conversion to sulphuric anhydrid in the converter.

In known processes for making contact sulphuric acid by the use of platinum contact material as a catalyst, the raw material for the production of the S02 gas mixture must be brimstone of the Louisiana type having a very minute arsenic content (about .0002% of A5203); otherwise the process has been found to be inoperative through poisoning of the platinum catalyst from the arsenic.

According to my invention, I have discovered a novel process for making contact sulphuric ajcid,.or rather for converting an S02 gas mixture (7 to 8% S02) into sulphur trioxide, while maintaining the proper temperatures throughout the entire conversion with a gas mixture derived from raw materials such as impure brimstone or Spanish pyrites ore containing at least five times the arsenic content of brimstone of the Louisiana type, along with chlorine, fluorine and other impurities, such as dust and fume that are ordinarily present when sulphide ores containing zinc, lead and arsenic are used as the raw material forthe production of the S02 gas mixture. The other impurities above mentioned and dust may'consist of such materials as selenium, silica and iron oxides. That the treatment of such impure raw material to produce therefrom sulphuric acid represents a real problem, will be readily appreciated when it is recognized that Spanish pyrites ore, for instance, contains only about 45% sulphur, the rest being extraneous material including lead, arsenic, chlorine, fluorine, and other impurities as indicated above. A representative analysis of this type of ore is given in Sulphuric Acid-Raw Material, byWyle (1923) on page 92. In my new process I introduce a purification step' in removing the major portions of the impurities to avoid coating or poisoning of the catalyst, deterioration of the catalyst carrier, and contamination of the product, and when the burner gas contains a large amount of zinc or lead fume, it is desirable to use a filtering device in.the passage of the gas before the latter enters the converter.

I have discovered that by using a catalyst of the vanadium type such as the so-called Sel'den catalysts which are generally silicates containing two or more bases more particularly describedin the U. S.Jaeger Letters Patent Nos. 1,675,308 and 1,675,309 of June 28, 1928, and No. 1,694,123 of December 4, 1928 arranged preferably in a converter or series of converters of the self-cleaning type such as are described in my pending application for patent Serial No. 390,510 filed September 5, 1929, I can obtain 97-98% conversion from the burner gases derived from raw materials having an arsenic content of at least five times that of brimstone of the Louisiana type and also when such gases contain chlorine, fluorine and other impurities. r

I have further discovered that with vanadium catalysts of the type described, a satisfactory conversion can be made without blocking or poisoning the catalysts provided the burner gases containing S02 are maintained at proper temperatures throughout the successive steps of the process with removal of dust and fume, so that my improved process may be advantageously employed in the wide field where impure raw materials are used to produce the burner gas containing sulphur dioxide. I am unable to state definitely why this relatively large arsenic content (when compared with brimstone of the Louisiana type), together with chlorine, fluorine and other impurities ordinarily found in the raw materials, fails to injure or poison these vanadium catalysts, but it may be due to the fact that the impurities volatilized from the raw material or carried over in the form of dust, such as selenium, arsenic, chlorine and fluorine are in contact with dust particles of iron oxides and silica at a high temperature, and that reactions take place between these impurities when maintained at a high temperature. This may result in their more ready removal from the gas stream by the purification process herein described. For example, it is well known that arsenic reacts with iron oxide to form a non-volatile product which can be readily removed with the dust from the gas stream. Likewise, fluorine and hydrofluoric acid reacts with silica to form products much more readily removable as such than the fluorine or hydrofluoric acid itself. Moreover, if the hydrofiuoric acid is permitted to enter the catalyst chamber, it will in time cause detrimental deterioration of the silicious catalyst carrier. Such removal under these conditions prevents coating or poisoning of the vanadium catalyst and likewise deterioration of the catalyst carrier.

Undoubtedly the mechanical eifect of the dust and fume is largely overcome when using a selfcleaning convertersuch as is described in my Isaid pending application Serial No. 390,510 in which the heat generated may be removed the manner described. i In setting forth my invention, I will make reference to the accompanying flow-sheet drawing diagrammatically illustrating a contact sulphuric acid plant in which my'irnproved process may be advantageously practiced. 7

In this drawing, the burner A'may be of a type suitable to the burning of sulphur contain ing ores, or of a type for burningbrimstone under.

the conditions WhlChI will describe. In this burner A, a gas is formed which will be about a 7% S02 gas having a'temper'ature of about 900 F.',-and the temperature of such gas leaving the burnerwill be indicated as 900 F. The air required to support 7 combustion the burner A. 1

and to form pa'rt'of the gas therein is drawn into the burner A by any suitable type of blower The' air entering the burnerA may be partially or completely driedbefore it enters the burner if desired. The hot gases from the burner A are then passed through a. dust collecting apparatus 7 B. Thisdust collector B ,may bejof any type 7 suitable for precipitating mostoi the dust in the gas. The gas leaving, the dust apparatus B will have atemperature of about 600 F. under normal operating conditions, though byinsulating the fines from the burner A to the dust apparatus B, and where necessary the dust apparatus,

the heat in the burner gas may beso conserved that the gases leaving the dust chamber Bare around750F. i ,The gases leaving the dust apparatus B are conducted to a. converter or series of converters C; This converter C may be oi the self-cleaning type described in my said pending application Serial No. 390,510.. The catalyst preferably used in the converter is of the "SeldenTvanadium type which I have hereinbefore mentioned. I

The hot gases) from the dust chamber B at approximately'750" F. so that hazardous condensation of sulphuric'acid is minimized, enter the converters or contact chambers C in which the mixed: S02 and air passing over, or through, the

vanadiumcontact material is converted to S03 7 e to the extent of 97-98%. 50.

. system, is about 800 F. N V

; These gases may be cooled by passing same.

The heat generated in the reaction may be removed in any manner,

preferably as shown in my copending application SerialNo. 390,510. The temperature of thegases at this point, i. e" where they leave the converter through a cooler, but will generally becooled sufficientlyiby the time they pass through the f flue to the acidcondenser D,' so that no separate piece of apparatus for cooling is required.

denser'D at a temperature around 600 F. or

' The gases from the converter Center the con-1 above thecondensation. point of sulphuric acid.

The condenser D may comprise theusual type of V absorber over which 99% sulphuric acid is circulated', or a preferred method of operation is to circulate acid of around 93% strength over this absorberu :Under the latter procedure the greater part of the S03 will be hydrated and condensed andlthebalance will pass along as a mist to the mist remover E. Water may be addedto further.

. humidify' the gases after the condenser 11. The

mist remover E in cooperation with the condenser 7 D are operated asdescribed in my PatentNo.

1,896,287 of February '7, .1933, so that the S03 gas will contactwith sulphuric acid having; a higher tiontemperaturesr aqueous vapor pressure than 08% acid'to partly absorb the S03 and. also humidify the balance of the S03 to be thereafter condensed'in the mist remover E and recovered. Leaving the mist remover E the gases passto the blower or fanF and are then discharged to the air..

During the entire passage of the gases from the burner A to the condenser D their temperature is conserved and maintained ata point where a considerable portion of the impurities will react to form non-Volatile products which are subsequently removed either in the dust apparatus B or in a subsequent filter. Thepresence of either a dust precipitator or remover and/or a filterper- 7 a prolonged'contact between the reacting impurities and gives them ample time to befconverted into'a physicaland chemical form'where their removal may be more readily accomplished.

In short, 'the'gases are never at any point permitted to become cool so that any, substantial reheating is needed to-bring theminto proper con- 7 dition for thexfurther treatments jAll heat changes required in thisprocessare of reducing temperatures. and not" of raising temperatures.

The gases are therefore maintained from the burner A through: to the'last converter'C atten i peratures approximating the. temperatures involved the original creation of the'LSOi gas mixture. {By my improved process theusual elaborate purifying equipment heretofore required when burning sulphide ores orlimpurebrimstone for producing the SOaiburner gas is avoided; i

. Where desired, iron oxides and/or silicainay be deliberately added to such S02 containing arsenic and fluorine and the mixture'maintained at ail-impurity reaction temperature to form such readily removable productswhere such gases do not already contain'the necessary react ing ingredients. a f V I t I I ilclaim;

r 1. In the process of making'contactsulphuric acid, the steps which comprise;burning' sulphur containing material to produce a sulphur dioxide gas mixture containing" fluorine, introducing;

sufiicient silica into the gas to provide a content thereof sufficient to'react with all the fluorine contact process which comprises burningsulphide ores containing relatively'large amountsof impurities including halogens and arsenic Imder' conditions such that a hot sulphur dioxide gas to reacting contactwith a'catalyst of" the V I mixture is obtained containinggaseous halogen and arsenic compoundsand iron oxide, maintain ing the hot gasmixture so' obtained attemperav tures above 600"" Ftfor asumcient length of time to permit the gaseous-impurities to'r'eact; with suspended solid material including the iron. oxide,

subsequently removing dust, fume and n'on-vola tile constituents without material reduction of gas temperature, and passing theresulting gases througha catalystof the vanadium type at reac- 3. A process'of making contact process which comprises burning'sulphide sulphuric acid by' the' 1 ores containing IelaLtivelylargeamounts of impurities including halogen .and arsenic [under conditions such that a hot sulphundioxide' mix-9 ture is obtained containing gaseoushalogen and arsenic compounds, iron oxide and silica, maintaining the hot gas mixture so obtained at temperatures above 600 F. for a sufiicient length of time to permit the gaseous impurities to react with suspended solid material including the iron oxide and silica, subsequently removing dust, fume and non-volatile constituents without material reduction of gas temperature, and passing the resulting gases through a catalyst of the vanadium type at reaction temperatures.

4. A process of making sulphuric acid by the contact process which comprises burning Spanish pyrites under conditions such that a hot sulphur dioxide gas mixture is obtained containing gaseous halogen and arsenic compounds, iron oxide and silica, maintaining the hot gas mixtures so obtained at temperatures above 600 F. for a sufficient length of time to permit the gaseous impurities to react with suspended solid material including the iron oxide and silica, subsequently removing dust, fume and non-volatile constituents without material reduction of gas temperature, and passing the resulting gases through a catalyst of the vanadium type at reaction temperatures. CYRIL B. CLARK. 

